This invention relates to a method and apparatus for storing and subsequently releasing thermal energy. More particularly, this invention relates to a method and apparatus for storing and subsequently releasing thermal solar energy.
Prior to the present invention, a wide variety of salt hydrates have been utilized to store heat for subsequent extraction upon demand. These salt hydrates are useful for this purpose since each is characterized by a relatively high heat of fusion but the phase change between solid and liquid in each occurs at a different temperature within a moderate range of temperatures. Heat can be stored both as sensible heat and as the latent heat of fusion of the selected salt hydrate for subsequent release during crystallization of that salt hydrate at its usual phase-change temperature by heat exchange with any of a variety of heat exchange liquids.
Significant problems associated with all the salt hydrates have greatly limited their use as heat storage media. Many salt hydrates are prone to super-cooling so that the phase change from liquid to solid does not readily occur and the latent heat of fusion is not recovered when desired. This phenomenon has necessitated adding nucleating agents to the salt hydrates to minimize supercooling. Even the presence of nucleating agents does not assure that the salt hydrate will crystallize upon cooling. The salt hydrates also have a tendency to become dehydrated gradually when exposed to repeated phase-change thermal cycling. Dehydration results in the salts developing different densities dependent upon the degree of hydration, with accompanying separation and stratification of the salts. When this separation occurs, it becomes increasingly difficult to cause the salts to undergo phase change concurrently since they have developed different melting points. Thus, some of the salts in a container will not undergo a phase change and may not release the latent heat of fusion that otherwise would have been extracted during a given thermal cycle. It has been proposed also to suspend the salts in gelatinous types of medium to overcome the separation problem. However, this reduces the thermal capacity of the resultant composition on a volume basis and thereby reduces its heat exchange effectiveness.
It has also been proposed to agitate a container of the salt hydrate to prevent or minimize supercooling at or near the temperatures at which the salt hydrate undergoes phase change. In these methods, a heat exchange fluid is passed continuously into heat exchange relationship with the agitated container and then either is directed back to the source of heat or is directed to the area of ultimate use. In these proposals, the salt hydrate is the sole means for thermal storage while the heat exchange fluid is used solely to carry heat to the area of thermal demand or to transfer heat from the thermal energy source to the salt hydrate and this requires relatively high mass flow rates of heat exchange fluid, effective heat exchange surfaces and large amounts of salt hydrate.
Accordingly, it would be desirable to provide a means for storing thermal energy based upon the use of salt hydrates which avoids the problem of supercooling, which minimizes or prevents salt separation and modification caused by repeated thermal cycling and which provides low cost, effective heat exchange between the salt hydrate and the heat exchange fluid.